The subject invention is primarily suited for use with alkaline type cells. Standard alkaline cells are conventionally manufactured with a cathode which generally comprises predominantly an oxide depolarizer, such as manganese dioxide usually admixed with a binder and conductive material such as graphite and the like. The cathode is generally pressed firmly against the inside wall of a container with the inner portion of the cathode left hollow so that the cathode assumes a closed end cylindrical structure with a centrally defined cavity. Separator material, usually formed into thin sheets of paper-like material, is inserted into the cathode's central cavity and conforms to the cavity of the cathode. An anode which usually comprises a consumable anodic material such as powder zinc admixed with a gelling agent such as polyacrylic acid or carboxymethyl cellulose and a suitable alkaline electrolyte such as an aqueous potassium hydroxide solution, is then extruded into the cavity of the separator. Thus the sheets of separator material electrically isolates the anode from the cathode while simultaneously permitting ions to flow between the electrodes. Generally, the separator strips can be forced downward and through the cathode's central cavity so that the separator's center region is parallel with and close to the bottom of the container. Occasionally, the insertion of the separator, using a rod driving means, forces the center region of the separator into the cathode material at the bottom of the container. On occasions, small pieces of the cathode are driven into and partially through the separator. This can result in short circuit of the anode and cathode of the cell thus rendering the cell useless. In addition, sometimes during discharge of the cell, zinc dendrites are formed that can extend through the separator at the bottom region of the container and short circuit the cell. These problems are difficult to detect because the cell would have to be disassembled in order to inspect the bottom central portion of the separator for evidence of cathode mix penetration or dendrite penetration. However, this problem has been substantially eliminated by placing an electrically insulating barrier, such as a plastic film, on the bottom surface of the separator. The use of an electrically insulating barrier at the bottom surface of the separator is disclosed in copending patent application number U.S. Pat. No. 560,651 by Kelemen et al filed on July 30, 1990.
British Patent 2,181,584 describes a method of producing a separator basket for standard alkaline batteries. This process uses a mandrel to form and support a tube of separator material on which the separator material is folded over one end of the tube and then a small amount of sealant is dispensed on the outside surface of the folded over bottom of the separator to form a "basket". The sealant seals the end of the separator basket and in some applications may secure the separator to other cell components such as the cathode. A final step may include insertion of a can containing a molded cathode over the mandrel-supported separator.
However, there is a problem associated with using strips of separator material to form a separator basket to be used in cylindrical type alkaline cells. Specifically, one problem in this type of basket separator is that undesirable folds are formed in the separator strips near the bottom segment of the basket. These projected folds or protrusions are inherent to a process of forming a retaining basket or cavity by inserting two rectangularly shaped strips of material into a circular opening so that the strip segments are aligned and symmetrically positioned within a tubular shaped electrode (cathode). The undesirable folds occur where the inner most surface of the separator contacts the bottom area of the electrode. These protrusions of folded material, commonly referred to as "ears", may extend to the inside and/or outside of the separator basket. These protrusions are an unavoidable part of a process which forces strips of uniform width into a circular type opening whose inside diameter is less than the width of the strips.
At least two problems can be caused by the presence of these protrusions. In alkaline cells, if the anode mix flows into the gaps or folds of the protrusions, the zinc in the anode may work its way between the strips and eventually cause an electrical short within the cell. The second problem pertains to folds in the separator that protrude into the retaining cavity thereby preventing the anode from falling to the bottom of the retaining cavity. This problem can force the trailing end of the anode to project above the top of the cathode and thereby create a situation in which the anode can be easily smeared onto the cathode during the remainder of the cell assembly process. In addition, the removal of the excess protrusion material will permit more active material to be placed into the cell and thereby provide greater output capacity.
It is an object of the present invention to provide a separator basket for electrochemical cells that is made from separator strips of material and wherein said separator basket has no effective protrusions or folds thereby providing a more uniform cavity.
It is another object of the present invention to provide a separator basket made from rectangular strips of material which have notches appropriately formed in the edges of the strips so that the strips upon being assembled to form a separator basket will effectively be free of any undesirable folds protruding from the wall of the basket.
It is another object of the present invention to provide a separator basket that is easy to make and cost effective to produce.
It is another object of the present invention to provide a process for assembling the separator basket into a cell.
These and other objects and advantages of the invention will be apparent from the following description and drawings.